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In the previous post we introduced code signing as implemented in Android and saw that it is practically identical to JAR signing. Android requires all installed packages to be signed and makes heavy use of the attached code signing certificates in its security model. This is where the major differences with other platforms that use code signing lie, so we will explore the topic in more detail.
Java access control
Before we start digging into Android's security model, let's go through a quick overview of the corresponding features of the Java platform. Java was initially designed to support running potentially untrusted code, downloaded from a public network (mostly applets). The initial applet sandbox model was extended to a more flexible, policy-based scheme where different permissions can be granted based on the code's origin and author. Code origin refers to the place where classes are loaded from, typically a local file or a remote URL, while authorship is asserted vi…